1. Field of the Invention
The present invention relates to a dielectric ceramic composition having reduction-resistance, and an electronic device, such as a multilayer ceramic capacitor using the dielectric ceramic composition.
2. Description of the Related Art
Multilayer ceramic capacitor as an electronic device is required to have a high relative dielectric constant, a long lifetime of insulation resistance (IR), and a good DC bias characteristic, i.e. less relative dielectric constant change over time, and further, a good temperature characteristic. Particularly, depending on a purpose of use, temperature characteristic is required to be stable under a severe condition. Recently, multilayer ceramic capacitor is used in various kinds of electric devices, such as engine electronic control unit (ECU) loaded in the engine room, crank angle sensor, antilock break system (ABS) module, and so on. These electronic devices are used for a stable engine control, drive control and break control. Therefore, temperature stability of circuit is required to be good.
It can be expected that temperature of an environment wherein said electronic devices are used will drop to around −20° C. or less at cold districts in winter time, and further, after starting the engine, the temperature will rise to around 130° C. or more in summer time. Recently, there is a tendency to reduce wire harness which tie an electronic device to its control device, and that electronic devices may be set outside the car and environments of electric devices are becoming more severe. Therefore, capacitor used for said electric devices is required to have a stable temperature characteristic within a wide temperature range. Concretely, dielectric ceramic composition wherein temperature characteristic of the capacitance is required not only to satisfy X7R characteristic (−55 to 125° C., ΔC/C=within ±15%) of EIA standard, but to satisfy X8R characteristic (−55 to 150° C., ΔC/C=within ±15%) of EIA standard.
Some proposals were give for dielectric ceramic compositions those satisfy X8R characteristic.
The patent article 1 (the Japanese Unexamined Patent Publication No. H10-25157) and the patent article 2 (the Japanese Unexamined Patent Publication No. H09-40465) disclose a proposal to satisfy X8R characteristic by substituting Ba of BaTiO3 with Bi, Pb or so and shift Curie temperature to a higher temperature. Further, a proposal to satisfy X8R characteristic by selecting a composition of BaTiO3+CaZrO3+ZnO+Nb2O5 is further disclosed. (the patent article 3 (Japanese Unexamined Patent Publication No. H04-295048), the patent article 4(Japanese Unexamined Patent Publication No. H 04-292458), the patent article 5 (Japanese Unexamined Patent Publication No. H04-292459), the patent article 6 (Japanese Unexamined Patent Publication No. H05-109319), the patent article 7 (Japanese Unexamined Patent Publication No. H06-243721))
However, since any of said composition uses Pb, Bi, and Zn that are likely to evaporate and fly, it is necessary to fire in an oxidizing atmosphere such as air. Therefore, there is a problem that an inexpensive base metal such as Ni cannot be used in inner electrode of capacitor, and an expensive precious metal such as Pd, Au, and Ag must be used in said inner electrode.
To overcome the problems, the applicant has already proposed the below-mentioned dielectric ceramic composition with an object to provide a high dielectric constant, a satisfactory X8R characteristic, and to be able to fire in a reduced atmosphere. (the patent article 8 (Japanese Unexamined Patent Publication No. 2000-154057).
Dielectric ceramic composition as described in the Patent Article 8 at least comprises BaTiO3 as main component, the first subcomponent selected at least one kind from MgO, CaO, BaO, SrO and Cr2O3, the second subcomponent expressed by (Ba, Ca)xSiO2+X, (note X=0.8 to 1.2), the third subcomponent selected at least one kind from V2O5, MoO3, and WO3, and the fourth subcomponent comprising R1 oxide (Note R1 is selected at least one kind from Sc, Er, Tm, Yb and Lu), and ratios of the respective subcomponents with respect to 100 moles of said main component are the first subcomponent: 0.1 to 3 moles, the second subcomponent: 2 to 10 moles, the third subcomponent: 0.01 to 0.5 mole, the fourth subcomponent: 0.5 to 7 moles (note that the number of moles of the fourth subcomponent is a ratio of R1 alone).
Further, the applicant already has proposed below-mentioned dielectric ceramic composition (the patent article 9(Japanese Unexamined Patent No. 2001-192264)).
Dielectric ceramic composition as described in the Patent Article 9 at least comprises Barium Titanate as main component, the first subcomponent selected at least one kind from MgO, CaO, BaO, SrO and Cr2O3, the second subcomponent comprising silicon oxide as main component, the third subcomponent selected at least one kind from V2O5, MoO3, and WO3, the fourth subcomponent comprising R1 oxide (Note R1 is selected at least one kind from Sc, Er, Tm, Yb and Lu), and the fifth subcomponent comprising CaZrO3 or CaO+ZrO2, and ratios of the respective subcomponents with respect to 100 moles of said main component are the first subcomponent: 0.1 to 3 moles, the second subcomponent: 2 to 10 moles, the third subcomponent: 0.01 to 0.5 mole, the fourth subcomponent: 0.5 to 7 moles (note that the number of moles of the fourth subcomponent is a ratio of R1 alone), the fifth subcomponent: 0<the fifth subcomponent≦5 moles.
However, with the dielectric ceramic compositions of said applications, it was found by the applicants that, it was difficult to satisfy X8R characteristic of the capacitance, and insurance resistance lifetime tends decrease when dielectric layer was further laminated. Considering temperature characteristic of the capacitance, capacitance change rate tends to increase particularly at higher temperature and it is expected to improve. Further, since rare-earth oxide including lanthanoide series element is expensive, inexpensive substituting element that can obtain the same characteristic has been searched. Furthermore, a tendency for integration and higher density of circuit is becoming more and more strong and a demand for a small sized mass-capacitance capacity is increasing and that inner dielectric layer is required to be more thinner.
In order to satisfy these demands, the applicants already have proposed below-mentioned dielectric ceramic composition (Patent Article 10 (Japanese Patent Article No. 2002-255639)). Dielectric ceramic composition as described in the Patent Article 10 at least comprises barium titanate as main component, the first subcomponent comprising AE oxide (Note AE is selected at least one kind from Mg, Ca, Ba and Sr), the second subcomponent comprising R oxide (Note R is selected at least one kind from Y, Dy, Ho and Er), and ratios of the respective subcomponents with respect to 100 moles of said main component are the first subcomponent: 0 mole<the first subcomponent<0.1 mole, the second subcomponent: 1 mole<the second subcomponent<7 moles.
With said patent article 10, X8R characteristic can be satisfied, however, IR temperature dependency from room temperature to higher temperature deteriorates and it was difficult to use them as a product.
Conventionally, in order to secure high dielectric constant and good temperature characteristic in dielectric ceramic composition comprising barium titanate as main component, core-shell structure having a ferroelectric phase part(core) and paraelectric phase part(shell) surrounding said core in crystal particle is thought to be useful. Various proposals have been made from the past for the core-shell structure of crystal particle.
For instance, patent article 11 (Japanese unexamined Patent Application No. 10-308321) proposes a method to control dispersing depth of Mg from the surface to the inner side of crystal particle within a fixed range, by adding Mg as a subcomponent to barium titanate as a main component and adjusting the firing temperature and firing time. With this method, dielectric ceramic composition having sufficiently high dielectric constant and good temperature characteristic can be obtained and quality level of multilayer ceramic capacitor can be improved.
With the method of the patent article 11, dielectric constant was high and temperature characteristic was good to 125° C., however, bad to 150° C. and IR temperature dependency from room temperature to higher temperature deteriorates and it was difficult to use them as a product.
Further, dielectric ceramic composition wherein Ca is added and as a subcomponent to main component of barium titanate, and all of said Ca is solved in said main component, is well known (See Patent Article 12 (Japanese Unexamined Patent No. 2003-48774)). However, with the method of the patent article 12, since Ca is all solved in barium titanate, good temperature characteristic could not be shown even-dielectric constant was high.